1. Field of the Invention
The present invention relates to a thin-film capacitor and, particularly, to a thin-film capacitor having a large capacity and a low inductance arranged, for example, in an electric circuit that operates at high speeds, and works for by-passing high-frequency noise or for preventing a change in the power source voltage, and to a board equipped with such a thin-film capacitor.
2. Description of the Prior Art
Accompanying the recent trend toward realizing electronic equipment in small sizes yet maintaining highly sophisticated functions, it has been strongly urged to provide electronic parts having small sizes and reduced thicknesses so that they can be installed in the electronic equipment to cope with the demand for high-frequency use.
In a high-speed digital circuit of a computer that must process large quantity of data at high speeds, in particular, the clock frequency in a CPU chip is as high as from 100 MHz to several hundred MHz and the clock frequency of a bus among the chips is also as high as from 30 MHz to 100 MHz even on a level of a personal computer.
As the degree of integration of LSI increases and the number of elements increases in the chip, further, the power source voltage must be lowered to suppress the consumption of electric power. As the IC circuits are highly densely arranged to operate at high speeds at low voltages, passive parts such as capacitors must also be designed in small sizes yet having large capacities to exhibit excellent characteristics for high-frequency pulses or high-speed pulses.
In order to realize a capacitor of a small size yet having a large capacity, the most effective method is to decrease, like a thin film, the thickness of the dielectric held by a pair of electrodes. The thin film meets the above-mentioned trend toward lowering the voltage.
Problems that arouse accompanying the high-speed operation of the IC circuit are more serious than realizing the individual elements in small sizes. Among them, what is particularly important in removing high-frequency noise, which is the role of a capacitor, is the function for suppressing an instantaneous drop in the power source voltage that occurs when the logic circuits are simultaneously switched over by instantaneously supplying energy stored in the capacitor. The capacitor having such a function is a so-called decoupling capacitor.
Performance required for the decoupling capacitor is to how quickly supply an electric current in response to a change in the current in the load unit that varies faster than the clock frequency. Therefore, the decoupling capacitor must reliably work over a frequency region of from 100 MHz to 1 GHz.
In practice, however, the capacitor element includes a resistance component and an inductance component in addition to the electrostatic capacity component. The impedance due to the capacity component decreases with an increase in the frequency, but the impedance due to the inductance component increases with an increase in the frequency. As the operation frequency increases, therefore, the inductance possessed by the element limits the transient current that is to be supplied causing an instantaneous drop in the power source voltage on the side of the logic circuit or newly generating voltage noise. As a result, an error occurs on the logic circuit.
In modern LSIs, in particular, the power source voltage has been lowered to suppress an increase in the consumption of electric power that results from an increase in the total number of elements, and the width of permissible change in the power source voltage has been decreased. In order to minimize the width of change in the voltage of during the high-speed operation, therefore, it becomes very important to decrease the inductance possessed by the decoupling capacitor element.
There are three methods of decreasing the inductance. A first method is to minimize the length of the current path, a second method is to cancel the magnetic field formed by a current path by a magnetic field formed by another current path located close thereto, and a third method is to divide the current path into a number of n to decrease the effective inductance down to 1/n.
The first method is to decrease the size by increasing the capacity per a unit area, and is accomplished by realizing the capacitor in the form of a thin film. Japanese Unexamined Patent Publication (Kokai) No. 94716/1985 discloses a dielectric in the form of a thin film having a thickness of not larger than 1 .mu.m in order to obtain a capacitor having a large capacity and favorable high-frequency characteristics.
When a decoupling capacitor that can be mounted on a desired place is taken into consideration, however, the size must be larger than about 0.5 mm.times.0.5 mm so that it can be handled. According to the first method of decreasing the thickness of the film and decreasing the size, therefore, limitation is imposed on decreasing the inductance.
It can be contrived to use the above-mentioned first to third methods in combination without yet, however, making it possible to obtain a thin-film capacitor having sufficient characteristics from the standpoint of decreasing the size, decreasing the thickness, increasing the capacity and operating at high frequencies.